A grain-oriented electrical steel sheet is a steel sheet which contains Si and in which crystal grains are highly integrated in a {110}<001> orientation, and is used as a material of a wound core of a stationary induction apparatus such as a transformer. The control of the orientation of the crystal grains is conducted with catastrophic grain growth phenomenon called secondary recrystallization.
As a method of controlling the secondary recrystallization, the following two methods can be cited. In one method, heating is performed on a slab at a temperature of 1280° C. or higher to almost completely solid-solve fine precipitates called inhibitors, and thereafter hot rolling, cold rolling, annealing and so on are performed to cause the fine precipitates to precipitate during the hot rolling and the annealing. In the other method, heating is performed on a slab at a temperature of lower than 1280° C., and thereafter hot rolling, cold rolling, decarburization annealing, nitriding, finish annealing and so on are performed to cause AlN (Al, Si)N and the like to precipitate as inhibitors during the nitriding. The former method is sometimes called a high-temperature slab heating method, and the latter method is sometimes called a low-temperature slab heating method.
In the low-temperature slab heating method, nitridation annealing is normally performed after decarburization annealing also serving as primary recrystallization annealing is performed, and the decarburization annealing and the nitridation annealing are tried to be simultaneously performed in recent years. If it becomes possible to simultaneously perform the decarburization annealing and the nitridation annealing, it becomes possible to perform them in one furnace and use existing annealing facilities, and to reduce the total treatment time required for annealing and suppress the energy consumption.
However, simultaneously performing the decarburization annealing and the nitridation annealing causes a remarkable variation in magnetic property (magnetic property deviation) depending on site, after the finish annealing performed with the steel being coiled.